## Animal is-a object (yes, sort of confusing) look at the extra credit（附加练习）
class Animal(object):
    pass

class Dog(Animal):

    def __init__(self,name):
        self.name = name

class Cat(Animal):
    def __init__(self,name):
        self.name = name

class Person(object):
    def __init__(self,name):
        self.name = name
        self.pet = None

class Employee(Person):
    def __init__(self,name,salary):
        super(Employee,self).__init__(name)
        self.salary = salary

class Fish(object):
    pass

class Salmon(Fish):
    pass

class Halibut(Fish):
    pass

rover = Dog("Rover")

satan = Cat("Satan")

mary = Person("Mary")

mary.pet = satan

frank = Employee("Frank",120000)

frank.pet = rover

flipper = Fish()

crouse = Salmon()

harry = Halibut()
